One of the major tasks in mineral recovery involves the separation of the desired mineral from the ore in which it is contained. Froth flotation is one of the most common techniques used for this purpose. In this technique, crushed ore is placed into a froth flotation tank. The chemical and physical properties of the fluid in the froth flotation tank are adjusted such that the desired ore particles can preferentially attach to bubbles which are rising upward through the fluid in the flotation tank.
One of the major problems associated with the froth flotation technique is the inability of the technique to float relatively large particles. B. A. Wills, in "Mineral Processing Technology", Second Edition, pp. 316-370, Pergamon Press (1981), teaches that the froth flotation process can only be applied to relatively fine particles. If the particles are too large, then the adhesion between the particle and the bubble will be less than that of the particle weight and the bubble will drop its load. Wills also teaches the advantages of floating a mineral as coarse as possible. These advantages include; lower grinding costs, increased recovery due to decreased slime losses, fewer overground particles, increased metallurgical efficiency, less flotation equipment, and increased efficiency in thickening and filtration stages. While the particle size that can be floated will vary with the specific mineral type, Wills discloses that the upper size limit is normally about 300 micrometers in diameter.
Flotation systems are discussed by Richard R. Klimpel in an article entitled "Considerations for Improving the Performance of Froth Flotation Systems", Mining Engineering, pp. 1093-1100, December, 1988. In the article, R. R. Klimpel segments the flotation system into three groups of components. The first group, chemistry components, includes collectors, frothers, activators, depressants and pH. The second group, operation components, includes feed rate, mineralogy, particle size, pulp density and temperature. The third group, equipment components, includes cell design, agitation, air flow, cell bank configuration and cell bank control. Regarding particle size, Klimpel points out "the greater the amount of large or small particles, or of both large and small, the more difficult it is to achieve excellent flotation results" Klimpel recognizes that "significant departures from existing highly agitated, short-mean-path particle fall designs are necessary" in order to change certain flotation results. However, no specific cell designs are disclosed.
While many variations exist on the market, froth flotation machines can be divided into two general categories. These categories are the pneumatic machines and the mechanical, or sub-aeration machines.
In a pneumatic machine, air is used to produce a froth and create aeration. Further, the air maintains the suspension and circulates it. Hence, an excessive amount of air is introduced into the system to achieve both of these goals. Improvements have been made on the basic design. For example, in the Davcra cell, pulp is pumped into the bottom by a cyclone and is dissipated against a baffle. Dispersion of the air and collection of particles occurs in a highly agitated zone. The tailings exit the machine at the bottom. In a flotation column design, water moves downward with particles to contact the bubbles which rise upward into the froth to be removed. Again, tailings are removed from the bottom. The flotation column is said to work best on relatively fine particles.
In a mechanical, or sub-aeration cell, incoming air is divided into air bubbles prior to being diffused through the pulp. For example, in the Denver Sub-A machine, an impeller shears the air stream into fine bubbles, while simultaneously drawing pulp into the cell to intimately mix with the bubbles. The bubbles then rise in a quiescent zone, so agitation does not cause bubbles to drop their load. In the Denver DR machine, pulp is passed through a circulation pump while air is pressure fed through a casing pipe. The aerated pulp stream rises, and prior to reaching the froth layer level, the pulp passes through a circulation pump and returns to the impeller. In the Wemco-Fagergren cell, the impeller is replaced by a rotor-disperser assembly. Pulp is drawn into the rotor by a suction action, and air is drawn from above. The two are intimately mixed as the disperser breaks the mixture into smaller bubbles. These bubbles then rise into the froth and are removed from the system.
Conventional flotation systems typically do not provide the critical upward flow velocity required for the flotation of large particles. Typically, flotation systems rely on the adhesive forces between the bubble and particle to overcome the force of gravity pulling downward on the particle. As a result, relatively large particles are not able to be floated. Additionally, due to inadequate bubble attachment, prior art devices typically cannot float middling particles. As used herein, the term "middling" refers to those particles which contain minor amounts of desirable mineral attached to undesirable gangue. Because the bubbles selectively attach to the mineral in typical flotation processes, the area for attachment in a middling particle is relatively small. As a result, middling particles typically do not float well in prior art devices. Also, in some cases, collection of particles takes place in a highly agitated zone. As a result, particles are often knocked loose from the bubbles supporting them, resulting in loss of efficiency. Further, paddles must typically be employed to remove the froth.
Therefore, it would be advantageous to produce a flotation device with the ability to float relatively large particles as well as middling. Further, it would be advantageous to decrease the amount of agitation in the flotation zone. It would also be advantageous to produce a flotation device in which froth can be removed without the use of mechanical devices such as froth paddles.